SAN FRANCISCO — When rangers came across mummified wood uncovered by a melting glacier in the northernmost Arctic reaches of Canada, they had no idea they were staring at an ancient forest dating back millions of years. Researchers eventually found a twisted tangle of preserved trees that reflects a harsh struggle to survive during an ancient global cooling period.

The spindly trees would have barely hung on during a time when the Arctic climate changed from greenhouse to icehouse, on top of enduring darkness for half of each year. Signs of stress are evident in narrow tree rings and undersized leaves that were preserved at the time of death — when a landslide may have buried the trees alive.